This invention relates to a method for measuring NOx and more particularly to the NO resonance absorption method for measuring NOx.
The brochure CLD 70E/February 1997 produced by ECO-Physics discloses an analysis unit for measuring low NO concentrations which uses the chemiluminescence method (CLD). However, the equipment associated with this method has a number of drawbacks.
Firstly, an ozone generator is required in order to produce the ozone which is required for the chemiluminescence reaction. Ozone is highly toxic and a relatively high outlay is required in order to prevent ozone from escaping from the equipment. For example, an ozone annihilator is required to be arranged at the measurement gas outlet.
Secondly, the CLD method requires a vacuum pump with pressure regulation. A drawback of this is that the vacuum pump is susceptible to faults and involves high maintenance. A further drawback is the relatively high production costs of the CLD equipment.
A further brochure, produced by ABB Analytical, has disclosed the FTIR method, which can be used to detect low NO concentrations. In this case too, a relatively high outlay on equipment has to be incurred. Firstly, a long-path absorption cell has to be used in order to achieve the high measurement sensitivity. At the same time, the high volume of this cell prevents, for example, the rapid measurement which is desired when carrying out measurements in automotive exhaust gas. Secondly, chemical measurement calculations are required in order to suppress interfering cross-sensitivities. These calculations are complex and likewise hinder rapid measurement.
Finally, DE 2541162 has disclosed the photometric nitrogen oxide resonance absorption method. Although this method is simple to carry out and is highly selective for nitrogen oxide, the measurement sensitivity which has been achieved to date is insufficient to detect very low NO concentrations. Hitherto, this technique has employed a modulation method in which a uniformly revolving filter wheel produces a measurement phase and a comparison phase.
The actual measurement time in the measurement and comparison phases is relatively short with respect to the revolution time of the filter wheel. The unfavorable time ratio means that the optimum signal-to-noise ratio which is required is not produced and therefore nor is the optimum measurement sensitivity. Therefore it is desirable to increase the measurement sensitivity of the NO resonance absorption method.
In a device for measuring NOx having a specific discharge lamp as a beam source and a cuvette and a detector in a beam path from the beam source, an electrically rotatable filter wheel or shutter wheel to change various shutters or filters in a beam path from the beam source, a cuvette and a detector, a method for measuring NOx comprising the steps of:
(a) using an electronically rotatable filter wheel or shutter wheel in the beam path between the cuvette and the detector to change various shutters and filters between measurement and comparison phases; and
(b) controlling the filter wheel or shutter wheel to make the measurement time of the detector greater than the transfer time of the detector.
A device for measuring NOx comprising:
a. a specific discharge lamp as a beam source;
b. a cuvette and a detector in a beam path from the beam source;
c. an electrically rotatable filter wheel or shutter wheel for changing various filters or shutters in the beam path between the cuvette and the detector wherein the rotation of the filter wheel or the shutter wheel takes place in steps such that the measurement time of the detector is greater than the transfer time of the detector.
In a device for measuring NOx having a specific discharge lamp as a beam source and a cuvette and a detector in a beam path from the beam source, an electrically rotatable filter wheel or shutter wheel to change various shutters or filters in a beam path from the beam source, a cuvette and a detector, a method for measuring NOx comprising the steps of:
(a) using an electronically rotatable filter wheel or shutter wheel in the beam path between the cuvette and the detector to change various shutters and filters between measurement and comparison phases;
(b) controlling the filter wheel or shutter wheel by a stepper motor having a step width to make the measurement time of the detector greater than the transfer time of the detector; and
(c) determining the step width so that the position of the filter wheel or shutter wheel can be transferred directly between the measurement and comparison phases.